Firstly, well done on making Sketchfab the most prolific WebGL viewer out there, it's really helping show that WebGL is a feasible solution for cross-platform content!
It's great to see the API being opened up on Sketchfab, so that people might create tools / applications, even games with it.
With other alternatives out there ( WebGL viewer + JS control ), such as Blend4Web, PlayCanvas ( and even Unity to an extent, although their mobile support isn't great ), the cost of using the different technologies to develop an application for a client can be a factor in choosing the correct tech to use.
What costs would be involved in the following scenarios, using SketchFab...
1) Creating a simple game and releasing to anyone to play for free
2) Creating a simple branded game via an ad agency for a large commercial client, where it is released to anyone to play for free
3) Creating a game that generates revenue ( ads, in-app purchases if a login infrastructure etc exists )
4) Creating some interactive educational content, that schools pay for to use
5) Creating an online automated tool that auto generates a 3D interactive scene for clients, that uses a custom SketchFab as the end viewer.
6) Use the viewer without any Sketchfab branding ( replaced with no branding, or the clients branding ), or have the ability to turn off / add custom icons for additional functionality.
These are a few quick examples, with the aim of getting some sort of ball-park figures for using Sketchfab for interactive online 3D client projects.
Many thanks for any info you can give on this...